It is well-known that increasing the iodide content of silver iodo-bromide emulsions causes morphological modifications of the grains, as well as modifications concerning their intrinsic sensitivity and their disposition to chemical sensitization. For example, G. F. Duffin, "Photographic Emulsion Chemistry", The Focal Press, 1966, page 64, refers to the effect of iodide on the rapidity of grain growth, as well as to the increased photographic speed and improved spectral sensitization. Also, P. Glafkides, "Chimie-et Physique Photographique", Paul Montel, 1976, page 377, refers to the role of iodide in modifying the silver halide emulsion characteristics, mainly the photographic sensitivity, the necessary iodide content rarely exceding 5 moles percent.
In effect, when the iodide content excedes or equals 7-10 moles percent, the photographic sensitivity roughly decays.